Blood coagulation is a process consisting of a complex interaction of various blood components (or factors) that eventually gives rise to a fibrin clot. Generally, the blood components, which participate in what has been referred to as the coagulation “cascade”, are enzymatically inactive proteins (proenzymes or zymogens) that are converted to proteolytic enzymes by the action of an activator (which itself is an activated clotting factor). Coagulation factors that have undergone such a conversion are generally referred to as “active factors”, and are designated by the addition of the letter “a” to the name of the coagulation factor (e.g. Factor VIIa).
Initiation of the haemostatic process is mediated by the formation of a complex between tissue factor, exposed as a result of injury to the vessel wall, and Factor VIIa. This complex then converts Factors IX and X to their active forms. Factor Xa converts limited amounts of prothrombin to thrombin on the tissue factor-bearing cell. Thrombin activates platelets and Factors V and VIII into Factors Va and VIIIa, both cofactors in the further process leading to the full thrombin burst. This process includes generation of Factor Xa by Factor IXa (in complex with factor VIIIa) and occurs on the surface of activated platelets. Thrombin finally converts fibrinogen to fibrin resulting in formation of a fibrin clot. In recent years Factor VII and tissue factor have been found to be the main initiators of blood coagulation.
Factor VII is a trace plasma glycoprotein that circulates in blood as a single-chain zymogen. The zymogen is catalytically inactive. Single-chain Factor VII may be converted to two-chain Factor VIIa by Factor Xa, Factor XIIa, Factor IXa, Factor VIIa or thrombin in vitro. Factor Xa is believed to be the major physiological activator of Factor VII. Like several other plasma proteins involved in haemostasis, Factor VII is, like a number of other coagulation proteins, dependent on Vitamin K for its activity, which is required for the gamma-carboxylation of multiple glutamic acid residues that are clustered close to the amino terminus of the protein. These gamma-carboxylated glutamic acids are required for the metal ion-induced interaction of Factor VII with phospholipids. The conversion of zymogen Factor VII into the activated two-chain molecule occurs by cleavage of an internal Arg152-Ile153 peptide bond. In the presence of tissue factor, phospholipids and calcium ions, the two-chain Factor VIIa rapidly activates Factor X or Factor IX by limited proteolysis.
Thus, Factor VII has a domain structure comprising a domain rich in γ-carboxyglutamic acid residues (the “GLA domain”), a region containing sequences homologous to human epidermal growth factor, and a catalytic domain containing a serine protease catalytic triad. The catalytic domain is glycosylated in nature.
It is often desirable to stimulate or improve the coagulation cascade in a subject. Factor VIIa has been used to control bleeding disorders that have several causes such as clotting factor deficiencies (e.g. haemophilia A and B or deficiency of coagulation Factors XI or VII) or clotting factor inhibitors. Factor VIIa has also been used to control excessive bleeding occurring in subjects with a normally functioning blood clotting cascade (no clotting factor deficiencies or inhibitors against any of the coagulation factors). Such bleeding may, for example, be caused by a defective platelet function, thrombocytopenia or von Willebrand's disease. Bleeding is also a major problem in connection with surgery and other forms of tissue damage.
FVII can be prepared recombinantly, but the primary structure of Factor VII renders production of the functional protein in prokaryotic host cells impossible, since bacteria do not have the capacity to introduce the vitamin K-dependent γ-carboxylation essential to membrane binding of the protein. Therefore, production of FVII is restricted to expression in higher, mammalian cells. However, expression in mammalian cells is much more complicated and time-consuming than expression in prokaryotes, and the yields are as a rule more limited; in general production in mammalian cells is therefore more expensive than production using prokaryotic host cells.
European Patent No. 200,421 (ZymoGenetics) relates to the nucleotide sequence encoding human Factor VII and the recombinant expression of Factor VII in mammalian cells.
Dickinson et al. (Proc. Natl. Acad. Sci. USA (1996) 93, 14379-14384) relates to a Factor VII variant wherein Leu305 has been replaced by Ala (FVII(Ala305)).
Iwanaga et al. (Thromb. Haemost. (supplement August 1999), 466, abstract 1474) relates to Factor VIIa variants wherein residues 316-320 are deleted or residues 311-322 are replaced with the corresponding residues from trypsin.
Published international patent applications WO 01/83725, WO 02/22776, WO 03/027147, and WO 03/037932 and Danish patent application PA 2002 01423 all relate to variants of Factor VIIa with preserved or increased activity. WO 02/077218 relates to derivatives of Factor VIIa with prolonged serum half-life.